The invention relates to a shaft furnace, in particular a direct-reduction shaft furnace, with a charge composed of particulate material, in particular particulate material containing iron oxide and/or sponge iron, the said material being capable of being fed into the shaft furnace from above, and with, arranged in one plane, a multiplicity of gas-inlet orifices for a reduction gas in the region of the lower third of the shaft furnace, the shaft furnace being surrounded externally by an annular space which is connected to the gas-inlet orifices downwards by means of gas supply ducts.
Shaft furnaces, in particular direct-reduction shaft furnaces of the type described above, are known in many forms from the prior art. Such a shaft furnace, designed essentially as a cylindrical hollow body, contains, for example, a charge composed of particulate material containing iron oxide and/or sponge iron, the material containing iron oxide being fed into the upper part of the shaft furnace. By means of a plurality of gas-inlet orifices arranged over the circumference of the shaft furnace and located in the region of the lower third of the latter, a reduction gas emanating, for example, from a melt-down gasifier is injected into the shaft furnace and consequently into the solid charge. The hot dust-laden reduction gas flows upwards through the solid charge and, at the same time, reduces the iron oxide of the charge completely or partially to sponge iron.
The completely or partially reduced iron oxide is conveyed out of the shaft furnace by means of discharge devices arranged between the bottom region of the shaft furnace and the region of the gas-inlet orifices, the charge column located in the shaft furnace sinking downwards due to gravity.
A shaft furnace must, by virtue of its design, ensure that a uniform reaction course, which is as complete as possible, and uniform sinking of the charge material can take place in it.
AT B 387,037 discloses a shaft furnace for the thermal treatment of charge materials by means of gaseous media. In this case, for the supply of reduction gas, gas-inlet orifices are provided, which are covered by an annular skirt relative to the charge materials introduced in the shaft furnace. An annular cavity is provided between the annular skirt and an annular widening of the casing of the shaft furnace, so that the reduction gas introduced can be delivered to the charge materials so as to be distributed over the circumference of the shaft furnace.
This design of the gas supply system has major disadvantages. The inner walls of shaft furnaces are conventionally lined with refractory material, for example fireclay. However, such an annular skirt cannot be produced from individual fireclay bricks, since it is connected only via its upper circumference to the casing of the shaft furnace. In principle, however, this type of gas supply system is capable of being produced monolithically, that is to say so as to be manufactured from one piece. Nevertheless, for this purpose, individual segments of the shaft-furnace casing, together with that part of the annular skirt which is suspended on the said casing, would have to be manufactured in each case from a single piece of refractory material. It is scarcely possible for this to be carried out, however, because of the size of the segments and because of their complex geometry.
Furthermore, an annular skirt produced in this way would collapse during the first loading of the shaft furnace. The lateral forces arising from charges, for example due to process-dependent increases in volume, are considerable. The annular skirt would therefore break away outwards immediately.
German Patent 34 22 185 discloses an arrangement consisting of a gasifier and of a direct-reduction shaft furnace. The direct-reduction shaft furnace has, above its bottom, screw conveyers which are arranged in a star-shaped manner and by means of which particulate material is conveyed out of the shaft furnace. The inner ends of the screw conveyers are mounted in a conical fitting in the middle of the shaft furnace. This conical fitting is connected downwards to the melt-down gasifier, so that reduction gas can flow out of the melt-down gasifier through the conical fitting into the shaft furnace. Furthermore, reduction gas is supplied to the shaft furnace via at least one gas-inlet orifice which opens into an annular space formed by an annular skirt and the shaft-furnace casing. The same applies to this annular skirt as to that in AT B 387,037, that is to say it would immediately break away laterally and/or, on account of the abrazing forces of the charge moving past it, would be ground off. This is all the more relevant as the conical fitting located at the same height as the annular skirt constitutes, from the point of view of the charge material, a reduction in the free cross section of the shaft furnace. Consequently, the laterally effective forces arising from the charge in the region of the conical fitting and of the annular skirt are also substantially higher than in other regions of the shaft furnace. Moreover, in regions of reduced cross section the charge preferentially forms baked areas, agglomerations and bridges. This prevents the charge material from sinking uniformly.
The prior art, for example U.S. Pat. Nos. 3,816,101 or 4,046,557, discloses shaft furnaces, in which a reduction gas is first introduced into a cavity which annularly surrounds the shaft furnace and from which a plurality of gas supply ducts open into a frustoconical widening of the shaft furnace casing. This annular cavity has a rectangular cross-sectional surface in vertical section, and the gas supply ducts opening into the shaft furnace lead away from the bottom and/or from the inner wall of this annular space.
This gas supply system is unsuitable when the reduction gas is to be supplied so as to be distributed uniformly over the circumference of the shaft furnace. Since the charge material rests directly against each gas-inlet orifice, the number of points for the inlet of gas into the shaft furnace and therefore into the charge is only in each case as large as the number of gas-inlet orifices.
If a dust-laden reduction gas is used, dust may settle at the mouth of the gas supply ducts into the shaft furnace and reduce the gas permeability of the charge there, with the result that further dust settles, and so on and so forth, ultimately clogging the gas supply ducts. Further dust may also be deposited on the bottom of the annular space. In an extreme situation, even particulate material from the charge may pass into the annular space. It is not possible to remove the solids which have settled in the gas supply system, without decommissioning and emptying the shaft furnace. Faults in the passage of gas through the charge, which are caused by clogged gas supply ducts, lead to an uneven reduction of the charge material and a reduction in the product quality.
The object of the invention is, therefore, to provide a shaft furnace, in particular a direct-reduction shaft furnace, the gas supply system of which is designed in such a way that the disadvantages known from the prior art are avoided.
In particular, this gas supply system is to be capable of being produced in a simple way from conventional refractory material and is to have sufficient mechanical stability relative to the laterally acting forces arising from the charge. Dust-laden reduction gas is to be capable of being distributed uniformly on the circumference of the shaft furnace and therefore, as a further consequence, also in the charge, and the clogging of gas supply channels is to be avoided.
This object is achieved, according to the invention, in that the shaft contour has a diametral widening in the region of the gas-inlet orifices and the wall of shaft furnace is designed in such a way that an annular cavity is formed between the gas-inlet orifices arranged in the region of this diametral widening and the charge.
By means of the inventive design of the gas supply system, it is possible, for the first time, to supply gas to a shaft furnace so as to be distributed uniformly over its circumference, without the need to provide a mechanically unstable annular skirt which it is scarcely possible to produce from conventional refractory bricks.
According to another advantageous feature, a number of means for dividing the annular cavity into sections separated from one another are arranged in the region of the diametral widening and are fastened to or in the wall of the shaft furnace.
Of these means for dividing the annular cavity, for example 2 to 16, but preferably 4 to 8, are arranged essentially at an approximately uniform distance from one another in the region of the diametral widening, so that the annular cavity is subdivided into as many sections.
Preferably, these means for dividing the cavity are formed by vertically arranged metal sheets and/or plates which, in any event, are dimensioned in such a way that, in each case, such a means passes at least completely through the vertical cross section of the cavity.
According to a further advantageous embodiment, in addition to the means for dividing the cavity, further means for dividing the annular space into portions separated from one another are arranged in the annular space, gas being capable of being supplied from outside the shaft furnace, in each case independently, to each of the portions separated from one another.
The division of the annular cavity into sections separated from one another, together with the division of the annular space into portions separated from one another, proves advantageous, because it avoids or reduces the risk that, in the case of temporary faults in the passage of gas through the charge, the reduction gas will follow the path of least resistance and, as a result, reduction gas will flow through part-regions of the charge to an increased extent and other part-regions will be xe2x80x9cunder-suppliedxe2x80x9d with reduction gas.
Preferably, in this case, the means for dividing the annular space and the means for dividing the cavity are arranged in such a way that, in each case, a portion of the annular space is assigned to a number of sections of the cavity, with the result that gas can be supplied via the respective portion to the section or sections corresponding to it.
It is particularly preferred, in this case, that the number of means for dividing the annular space be equal to the number of means for dividing the cavity and that a portion be assigned in each case to one section.
Subdividing the annular space and the cavity by suitable means, for example refractory material, metal sheets, etc., gives rise to closed-off regions which can be subjected to gas quantities individually and in a controlled way. For example, it is possible, despite locally varying charge permeability, to introduce the same gas quantity into each region of the charge. It is, however, also possible, if the conduct of the process so requires, to introduce different gas quantities per region into the charge deliberately.
According to a further advantageous embodiment of the shaft furnace according to the invention, the vertical cross section of each portion of the annular space is designed to taper in the circumferential direction from the location of gas supply to the respective portion ends.
The result of this is that the velocity of the dust-laden gas from the location of gas supply as far as the respective portion end does not decrease or does not decrease as greatly as would be the case if the cross section of the annular space were constant in a circumferential direction. The gas velocity therefore remains sufficiently high at all the locations of the annular space, in order to avoid dust deposits in the annular space.
According to a further advantageous embodiment, to a number of gas supply ducts is assigned in each case a cleaning device which is capable of being operated from outside the shaft furnace and by means of which caked-on accumulations can be cleaned off from the gas supply ducts or from the annular space which precedes the gas supply ducts in the gas flow direction.
Process faults may also lead to deposits/caked-on accumulations in the annular space or the gas supply ducts. These deposits can be cleaned off by means of the cleaning device or cleaning devices. It is particularly advantageous that the cavity formed by the diametral widening affords a sufficiently large volume for receiving the released material, whereas, otherwise, this would lead merely to clogging of the gas supply ducts. Complicated shaft emptying or the outward extraction of the material is thus avoided.
In the simplest instance, in each case one cleaning device is expediently designed as a poker device, the poker device passing through the outer wall of the annular space essentially in each case in the extension of a respective gas supply duct.
According to a preferred embodiment, the diametral widening forms a frustoconical generated surface, the generatrix of which encloses with the horizontal an angle which is smaller than the angle of repose of the material located in the shaft furnace.
This results in the formation of an annular cavity which is delimited by the frustoconical generated surface, by part of the vertical inner wall of the shaft furnace and by the charge and in which the gas supplied through the gas-inlet orifices can be distributed uniformly. The term xe2x80x9cangle of reposexe2x80x9d is intended, in this case, to refer to the natural angle of repose which the generatrix of the generated surface of a charging cone forms with the horizontal.
Preferably, the angle which the generatrix of the generated surface encloses with the horizontal is 0 to 25xc2x0, whereby the diametral widening widens from the top downwards. The angle of repose of particulate sponge iron, ore pellets or particulate ore is about 35 to 40xc2x0. The difference between these two angles is therefore sufficiently large to give rise to an annular space, in which the reduction gas can be distributed optimally.
Particularly preferably, the angle which the generatrix of the generated surface encloses with the horizontal is 0xc2x0. In this design, the distance between the charge and the generated surface or the gas-inlet orifices arranged in the generated surface is such that the risk that dust-like or particulate material from the charge may pass into one of the gas supply ducts is minimized.
The gas supply system also has outstanding mechanical stability, since the dimensions of the gas supply ducts which pass through the wall of the shaft furnace can be kept so small that the gas-inlet orifices or the gas supply system formed by the gas supply ducts and by the refractory material surrounding the gas supply ducts can withstand the effective lateral forces arising from the charge.
The gas supply system is also capable of being produced in a simple way from conventional refractory material, for example fireclay bricks, since each part of the gas supply system is supported by parts located below it. No arrangements, such as, for example, an annular skirt, are provided, which would be connected to the wall of the shaft furnace solely via an upper edge.
As a result of an advantageous refinement, the gas supply ducts have an essentially rectangular cross section and are designed to taper from the bottom upwards, the inner edges of the gas supply ducts being rounded. This ensures that gas supply ducts, in which a build-up of material occurs in spite of the material-free annular cavity formed inside the shaft furnace, are cleaned again automatically, that is to say by means of the downward movement of the material in the shaft furnace.
According to a further advantageous refinement, the transition between the annular space, which externally surrounds the shaft furnace annularly, and the gas supply ducts is designed to descend obliquely downwards. Consequently, dust-like material from the reduction gas cannot accumulate in the annular space and, also, material which comes from the charge and which passes into the annular space due to process-induced faults cannot remain there. Instead, due to gravity, such material is returned to the shaft furnace again through the gas-inlet orifices which widen downwards.